Designers of microprocessor based systems are almost always seeking to increase the operating speed of the processor. Thus, in recent years, microprocessor specifications have offered ever-increasing operating speeds. However, for any given design of a microprocessor, manufacturing tolerances will affect the actual maximum speed at which the processor will operate reliably. Thus, particular microprocessor chips of a given design will likely have different maximum operating speeds. Accordingly, it is not uncommon in the industry to qualify microprocessor chips in several "grades" according to the maximum clock frequency at which they will operate. Thus, for example, certain chips of a single microprocessor design might be qualified for operation at a particular clock frequency, say 25 megahertz, while other chips might be qualified at lower frequencies, say 20 and 16 megahertz. Generally speaking, chips qualified at higher operating clock frequencies will command higher prices.
Typical microprocessor designs provide for a clock input to be provided by off-chip circuitry, usually comprising a crystal oscillator. Therefore, the microprocessor manufacturer has no direct control over the frequency of the clock signal that will be applied to the clock input of the microprocessor. While the maximum clock frequency of the microprocessor is specified by the manufacturer, a purchaser of a microprocessor chip may attempt to increase the performance of the microprocessor by applying a clock signal having a frequency higher than that which is specified as the maximum frequency. This can result in malfunction of the microprocessor and unpredictable data results. Such results may adversely reflect on the reputation of the microprocessor manufacturer. Accordingly, there is a need for a method and apparatus for governing the maximum operating speed of a microprocessor that is solely under the control of the chip manufacturer. Furthermore, there is a need for selecting one of a plurality of maximum operating speeds depending upon the qualification level of each particular chip.
It is also desirable for the microprocessor manufacturer to have some means for controlling the minimum frequency of the microprocessor. For example, a particular microprocessor design may incorporate a static mode whereby the clock signal may be removed altogether without disrupting operation of the microprocessor when the clock signal is restored. In some circumstances, it may be desirable to disable this particular feature to render the microprocessor "non-static" so that if the clock signal is removed and later restored, the microprocessor must undergo a system reset before operations can resume.